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how fast is f/2.8 compare to f/4.0?

as i know..... f/2.8 is about the double speed of f/4.0.....so you can use a shutter speed that is half with the f/2.8 compare to f/4.0....or you can compensate by doubling the ISO...is this doubling really makes a huge difference on speed?....because i saw many ppl saying that the sigma 18-55 f/2.8 is really fast......and nikkor VR 18-200 f/3.5-5.6 is slow....i just wonder why is that?.....nikkor VR 18-200 at 18-55mm range operates at f/4.0 or larger.....it is only half compare to f/2.8.....and giving twice as fast shutter doesnt really help with exposure much when the surrounding is dim....i've tried with my VR 18-200 in my living room....without flash...i often need to use shutter speed at less than 1/10 (aperature is at f/4.0)....which is a shutter speed almost impossible to shoot at.....i'm new to DSLR.....but do the lense make a difference even at the same aperature value?.......at the exposure value.....with the sigma at 55mm, 1/100, f/2.8 and nikkor at 55mm, 1/50, f/4.0......should i expect the same exposure result?...or i can double the ISO setting to obtain the same result?.....becuase i found that without a flash...often i'm looking at shutter speed in the 1/10 or slower range iwth ISO 400....so even with double aperature.....the shutter is still not fast enough.......my camera is D80 and my lense is nikkor VR 18-200 f/3.5-5.6

i would understand if we are working in the film world where changing film speed is a troublesome job...but with DSLR....we can compensate the aperature with higher ISO speed or slower shutter speed or combination of both.....isnt it?....i'm new to DSLR (new in photography)...please dont get offended by my stupidity....i'm just curious

For me, the 18-200VR is a great lens... However, yes, it is slow. I can't say that I have ever felt I had to "wait" for focus, but I have often had to lower my shutter speed. This is ok, if the subject is not moving, because the VR does very impressive work.

That said, I have better results shooting my son's basketball games with the Nikorr 55mm f/1.8 prime, and cropping to get the framing.

or i can double the ISO setting to obtain the same result?.....becuase i found that without a flash...often i'm looking at shutter speed in the 1/10 or slower range iwth ISO 400....so even with double aperature.....the shutter is still not fast enough

Doubling the ISO is not really going to give the same result because doubling the ISO is going to add more grain to the photo. And a slower focusing lens is still going to focus slower and slower yet, if at all, when low-level lighting is involved, which to me, translates into missed shots if any sort of target is fast moving in low-level lighting. Myself, I started with some slower Sigma lenses and eventually moved on to faster Nikkor F/2.8 lenses.

Yes. The 18-200 is actually a f/3.5 at wider angle. But even that is not very fast. It does make a big difference. And faster ISO does not help much, especially with the added noise.

In general, focus speed IS typically affected by the speed of the lens. The more light that comes through, the easier it is for the auto-focus system. I was trying to relay that I have not found the auto-focus system for the 18-200mm to hunt in low light. IOW the auto focus is quicker than I would expect, considering the 'speed' of the lens.

So in general terms you can assume that someone talking about fast glass, fast lens, etc. They are talking about the f-stop. As the larger the aperature (smaller number) is, the better the lens is at stopping action, the faster shutter speed you can use, etc.

---edit---

Meant to add that normally if a lens doesn't focus fast then it will be specified that the lens has a slow autofocus, or that the lens "hunts" for focus.

Depends on if your using the flash or not. The flash will allow you to use a slower shutter and still "stop" the action. The SB-800 is very good at this. I can shoot at 1/40 and still freeze motion.

Without flash, it would depend on how fast the subject is, so its hard to give a shutter suggestion, but I dont shoot slower than 1/125. Sometimes thats fast enough, sometimes its not even close... just experiment and you will start to learn different settings.

*EDIT* Forgot to mention, you should probably delete the double post of this thread that you started.

when you guys say "FAST"....you mean faster focus??....i'm getting that feeling that you guys are referring to focus speed?.....or you mean shutter?.....the f/2.8 has a faster focus capability?

A little of both since some of the Nikkor F/2.8 lenses do have silent wave motor technology--quieter and faster focusing.

Myself, I found it very aggravating and limiting trying to take action photos with an F/3.5+ lens in low-level lighting since the lens still needed to "hunt" since it was unable to focus as fast in lower-level lighting. To me, the flash also doesn't allow the shots to appear as natural, shots can still be missed if the flash has to recharge, and of course, the flash is limited by it's range not to mention, flash photography is not always allowed. That is one reason I prefer the F/2.8 lenses that are actually F/2.8 all the way throughout their range.

If possible, visit a local camera store and try out the lenses. Heck, ask the salesperson to run around the mall and try to take photos without a flash using your own camera body and their lenses--that's what I did prior to purchasing the 70-200mm F/2.8 VR. I usually take sample photos home and check them out first as well--as I did with the 17-55mm F/2.8.